Soil and Water Cycle

 The soil is an essential component of the hydrologic cycle- movements of water on or near the Earth’s surface. It receives precipitation from the atmosphere, rejecting some of it, which is then forced to run off into streams and rivers, and absorbing the remainder, which then moves downward to be either transmitted to the groundwater, taken up and later transpired by plants, or evaporated directly from soil surfaces and returned to the atmosphere.

Percolation of water through the soil to the water table not only replenishes the groundwater, it also dissolves and carries downward a variety of inorganic and organic chemicals found in the soil or on the land surface. Chemicals leached from the soil to the groundwater (and eventually to streams and rivers) in this manner include elements weathered from minerals, natural organic compounds resulting from the decay of plant residues, plant nutrients derived from natural and human sources, and various synthetic chemicals applied intentionally or inadvertently to soils.

Once chemicals are carried below the zone of greatest root and microbial activity, they are less likely to be removed or degraded before being carried further down to the groundwater. This means that chemicals that are normally broken down in the soil by microorganisms within a few weeks may move down through large macropores to the groundwater before their degradation can occur. This is especially problematic for coarse-textured soils that allow these contaminants to move quickly through the soil horizons and down into the groundwater. In some areas underground sources of drinking water may be contaminated with excess nitrates to levels unsafe for human consumption.

Studies of the movement of chemicals in soils and from soils into the groundwater and downstream bodies of water have called attention to another critical problem. Accumulation of chemical nutrients in ponds, lakes, reservoirs, and groundwater downstream may stimulate a process called eutrophication, which ultimately depletes the oxygen content of the water, with disastrous effects on fish and other aquatic life.

A possible solution to this problem can be an application of nitrogen fixers, instead of chemical fertilizers.

Nitrogen Fixation

 Next to plant photosynthesis, biological nitrogen fixation is probably the most important biochemical reaction for life on Earth. This process converts the inert dinitrogen gas of the atmosphere (N₂) to reactive nitrogen that becomes available to all forms of life through the nitrogen cycle. The process is carried out by a limited number of bacteria, including several species of Rhizobium, actinomycetes, and cyanobacteria (blue-green algae).

Globally, enormous amounts of nitrogen are fixed biologically each year. Terrestrial systems alone fix an estimated 139 million Mg. However, the amount that is fixed in the manufacture of fertilizers is now nearly as great.

The mechanism:

Regardless of the organism involved, the key to biological nitrogen fixation is the enzyme nitrogenase, which catalyzes the reduction of dinitrogen gas into ammonia.

 

N₂ + 8H⁺ + 6e^{–  nitrogenase (Fe, Mo)}       2NH₃ + H₂

The ammonia, in turn, is combined with organic acids to form amino acids and, ultimately, proteins.

NH₃ + organic acids                 amino acids                 protein

 

The site of N₂ reduction is the enzyme nitrogenase, a complex consisting of two proteins, the smaller of which contains iron while the larger contains molybdenum and iron. Several salient facts about this enzyme and its function are worth noting, for nitrogenase is unique and its role in the nitrogen cycle is of great importance to humankind.

1. Breaking the triple bond in N₂ gas requires a great deal of energy. Therefore, the process is greatly enhanced by association with higher plants, which can supply this energy from photosynthesis.
2. Nitrogenase is destroyed by free O₂, so organisms that fix nitrogen must protect the enzyme from exposure to oxygen. When nitrogen fixation takes place in root nodules, one means of protecting the enzyme from free oxygen is the formation of a compound, which binds oxygen in such a way as to protect the nitrogenase while making oxygen available for respiration in other parts of the nodule tissue.
3. The reduction reaction is end-product inhibited—for example, an accumulation of ammonia will inhibit nitrogen fixation. Also, too much nitrate in the soil will inhibit the formation of nodules.
4. Nitrogen-fixing organisms have a relatively high requirement for molybdenum, iron, phosphorus, and sulfur, because these nutrients are either part of the nitrogenase molecule or are needed for its synthesis and use.

Biological nitrogen fixation occurs through a number of microbial systems that may or may not be directly or indirectly associated with higher plants. Although the legume—bacteria symbiotic systems have received the most attention, recent findings suggest that the other systems involve many more families of plants worldwide and may even rival the legume-associated systems as suppliers of  biological nitrogen to the soil.

The presence of nitrogen-fixing species can significantly increase the nitrogen content of the soil and benefit nonfixing species, like higher plant. Such contributions can be useful in managing agricultural systems and should be taken into account when estimating nitrogen fertilizer needs for maximum plant production with minimal environmental pollution.